Croup is a clinical diagnosis, and laboratory and imaging studies are not necessary to make the diagnosis. Imaging studies may be useful in excluding other causes of stridor (such as foreign body aspiration) in situations where the history and physical examination are unclear or when the child’s symptoms do not respond to usual treatment.
The most common radiological sign of croup on anteroposterior chest radiograph is the “steeple sign.”34 This distinctive narrowing of the trachea in the shape of an inverted V is produced by the presence of edema in the trachea, which results in loss of the normal shoulder-like appearance of the subglottis. (See Figure 1.) Other causes of steepling on radiograph include epiglottitis, thermal injury, angioneurotic edema, and bacterial tracheitis. The majority of children with croup will have normal radiographs, and films should not be routinely obtained. If epiglottitis cannot be ruled out clinically, a lateral neck radiograph may be obtained, which may show the thickening of the epiglottis and aryepiglottic folds called the “thumb sign.” In croup, the radiological findings on lateral neck radiographs are variable and may be difficult to identify, as they are tied to the dynamics of the hypopharynx during inspiration and expiration.35 Patients with suspected epiglottitis or croup must always be accompanied and monitored during procurement of the radiographs,6 as upper airway obstruction may progress rapidly in children. If a child looks unwell or has signs of severe airway obstruction, emergent airway management should occur first and obtention of radiographs is contraindicated.
Laboratory studies such as complete cell count or blood culture are not useful when evaluating a child with croup, and viral cultures or rapid antigen testing are not routinely recommended. If intubation is attempted, trachea cultures should be obtained to rule out the presence of bacterial tracheitis and to guide antibiotic treatment.